Lubricating oil composition

ABSTRACT

A lubricating oil composition which has a sulfur content of 0.5 wt. % or less and which at least comprises a base oil and the following additive components a) to e): a) an ashless dispersant comprising an alkenyl- or alkyl-succinimide or a derivative thereof; b) an ashless dispersant comprising an alkenyl- or alkyl-succinic ester or a derivative thereof; c) a molybdenum complex of a basic nitrogen-containing compound; d) an alkaline earth metal-containing detergent having TBN of 10-400 mg KOH/g; and e) an anti-wear agent favorably employed for lubricating internal combustion engines operated using a low sulfur hydrocarbon fuel.

This application is a continuation of U.S. patent application Ser. No.13/629,837 which claims priority to Japanese Patent Application numberJP 2011-214116 which was filed on Sep. 29, 2011.

FIELD OF INVENTION

The present invention relates to a lubricating oil composition favorablyemployable for lubricating internal combustion engines such as dieselengines and gasoline engines. In particular, the invention relates to alubricating oil composition having a low sulfur content of about 0.5 wt.% or less, particularly about 0.2 wt. % or less, which reduces frictionoccurring in the internal combustion engines mounted to land-travellingautomotives using low sulfur hydrocarbon fuel and shows good hightemperature detergency.

BACKGROUND OF INVENTION

The sulfur contained in fuels such as gasolines or diesel fuels employedfor operating internal combustion engines is oxidized when it burns inthe engine to give sulfur oxide such as sulfuric acid. A portion of thesulfur oxide enter the lubricating oil employed for lubricating theengine, and the remaining portion is exhausted with other exhaust gases.The sulfur oxide exhausted with other exhaust gases causes environmentalpollution. Hence, the sulfur content in the fuel has been heretoforeunder severe regulation. The regulation on the sulfur content wasinitially applied to gasolines (fuels for gasoline engines). However,the regulation on the sulfur content has been also applied recently todiesel fuels having a higher sulfur content. The sulfur content in thediesel fuels has been recently regulated to be not higher than 0.0010wt. %.

As a result of the decrease of the sulfur content, the lubricating oilis less contaminated with sulfur oxide such as sulfuric acid which isproduced when the fuel burns. This means that the amount of theoverbasic metal-containing detergent in the lubricating oil can bereduced. The basic metal-containing detergent serves for neutralizingthe sulfur oxide in the lubricating oil.

When the fuel burns in the engine, not only the fuel, but also a portionof the lubricating oil burns in the engine. A gas produced by burntlubricating oil is also exhausted with other exhaust gas. Therefore, itis desirable to reduce the amount of the sulfur content of thelubricating oil.

By the way, the recently employed lubricating oils for internalcombustion engines comprise a base oil (hydrocarbon oil havinglubricating viscosity) and a variety of additives (i.e., lubricating oiladditives) such as a basic metal-containing detergent for neutralizingthe emigrated sulfur oxide, an oxidation inhibitor for keeping the baseoil functioning at elevated temperature from deteriorating, a dispersantfor dispersing soots produced by burning of fuels and residues producedby deterioration of the base oil and metal-containing detergent in thebase oil, an extreme pressure agent for preventing of seizure of thepiston and cylinder, a friction-reducing agent (friction modifier) forreducing friction occurring between the piston and cylinder.

Therefore, the phosphorus component and metal component contained in theadditive placed in the lubricating oil composition are decomposed whenthe lubricating oil composition burns and enter the exhaust gas in theform of phosphorus oxides and metal oxides. The phosphorus oxides andmetal oxides contained in the exhaust gas, are brought into contact withoxidative catalyst or reductive catalyst placed in the exhaustgas-cleaning apparatus and inactivate the catalyst.

For the reasons described above, it has been demanded to reduce thesulfated ash content, phosphorus content and sulfur content of thelubricating oil composition.

Patent publication 1 (JP 2002-53888 A) discloses a lubricating oilcomposition favorably employable for lubricating internal combustionengines operated using a fuel having a low sulfur content, particularly,diesel engines. The disclosed lubricating oil composition comprises abase oil having a sulfur content of 0.1 wt. % or less (preferably 0.03wt. % or less) and the following additives:

a) an ashless dispersant comprising an alkenyl- or alkyl-succinimide ora derivative thereof in an amount of 0.01-0.3 wt. % in terms of thenitrogen content;

b) a metal-containing detergent having a sulfur content of 3 wt. % orless and a TBN of 10-350 mg KOH/g in an amount of 0.1-1 wt. % in termsof the sulfated ash content;

c) a zinc dialkyldithio phosphate in an amount of 0.01-0.12 wt. % interms of the phosphorus content;

d) an oxidation-inhibiting phenolic compound and/or anoxidation-inhibiting amine compound in an amount of 0.01-5 wt. %,

the lubricating oil composition having a sulfated ash content of 0.1-1wt. %, a phosphorus content of 0.01-0.1, a sulfur content of 0.01-0.3wt. %, a chlorine content of 40 wt.ppm or less, and an organic acid saltderived from the metal-containing detergent in an amount of 0.2 to 7 wt.% (preferably 0.5-5 wt. %, more preferably 1.0-3 wt. %),

wherein the above-mentioned contents and amounts are expressed based onthe total amount of the lubricating oil composition.

Patent Publication 1 further discloses in the working examples that thedisclosed lubricating oil compositions show good high temperaturedetergency in the internal combustion engines.

Patent Publication 1 furthermore discloses that the lubricating oilcomposition may contain a molybdenum-containing compound which can serveas a friction modifier, an oxidation inhibitor as well as an anti-wearagent. The molybdenum-containing compound can be one of knownmolybdenum-containing compounds such as sulfoxymolybdenumdithiocarbamate, sulfoxymolybdenum dithiophosphate and molybdenumcomplexes of succinimide possibly containing sulfur.

Patent Publication 2 (JP 2004-149802 A) discloses a lubricating oilcomposition having a low sulfated ash content, a low phosphorus contentand a low sulfur content, preferably employable for lubricating internalcombustion engines (that is, engine oil), which comprises a base oil, anethylene carbonate post-treated ashless dispersant, a borated ashlessdispersant, an overbased metal-containing detergent and a phosphoruscompound. Patent Publication 2 further discloses that the disclosedlubricating oil composition may contain a molybdenum-succinimidecomplex. In the working examples of Patent Publication 2, it is shown byexperimental data that the lubricating oil composition shows goodoxidation stability as well as good anti-wear performance.

Patent Publication 3 discloses a low sulfated ash, low phosphorus, lowsulfur engine oil (engine oil composition) comprising a low sulfur baseoil, a boron-containing ashless dispersant, a molybdenum-containingfriction modifier, a metal-containing detergent such as sulfonate,phenate or salicylate, and a zinc dithiophosphate. Themolybdenum-containing friction modifier can be molybdenumdiorgano-dithiocarbamate, molybdenum diorgano-dithiophosphate,molybdenum carboxylate, and a trinuclear molybdenum compound such asMo₃S₇(dtc)₄ or Mo₃S₄(dtc)₄. In the working examples, it is shown by theexperimental data that the lubricating oil composition employing theabove-mentioned trinuclear molybdenum compound has particularly goodhigh temperature oxidation stability and a good friction reductiveperformance.

As is described above, it has been known that the molybdenum-containingcompound is particularly effective as a friction modifier to beincorporated into lubricating oil compositions.

As the known molybdenum-containing compounds, molybdenum diorganodithiocarbamate and molybdenum diorgano dithiophosphate are mostgenerally employed. However, both compounds have disadvantage problems,in that the former contains sulfur (S) in its molecular structure andthe latter contains phosphorus (P) in its molecular structure, and henceboth produce poisonous gases such as sulfur oxide gas and phosphorusoxide gas when the lubricating oil burns in the internal combustionengine, resulting in inactivation of the catalysts of the exhaustgas-clearing apparatus. As is explained above, Patent Publication 3describes that a trinuclear molybdenum compound such as Mo₃S₇(dtc)₄ orMo₃S₄(dtc)₄ shows a good friction-modifying function (friction-reducingfunction). However, these trinuclear molybdenum compounds containssulfur atoms in the molecular structure of its ligand moiety, i.e.,dithiocarbamate, dtc, and further contains other sulfur atoms directlyattached to the molybdenum atom, and hence, the trinuclear molybdenumcompounds contains a relatively large amount of sulfur in the compoundper se. Therefore, the trinuclear molybdenum compound is not favorablyemployed for incorporation into the lubricating oil composition ifreduction of the sulfur content in the oil composition is intended.

SUMMARY OF INVENTION

Accordingly, it is the object of the invention to provide amolybdenum-containing friction-reducing agent having no or a relativelyless amount of sulfur and phosphorus which is favorably employable inplace of the conventionally employed friction-reducing agents containingsulfur and phosphorus in relatively large amounts, such as molybdenumdiorgano-dithiocarbamate, molybdenum diorgano-dithiophosphate,molybdenum carboxylate, and a trinuclear molybdenum compound such asMo₃S₇(dtc)₄ or Mo₃S₄(dtc)₄.

The present inventors have found that the friction-reducing function ofthe known molybdenum complex of a basic nitrogen-containing compoundsuch as a molybdenum-succinimide complex is prominently enhanced if themolybdenum complex is incorporated into lubricating oil compositions incombination with specifically selected amounts of an alkenyl- oralkyl-succinimide or its derivative (which is known as an ashlessdispersant) and an alkenyl- or alkyl-succinic acid ester (succinate) orits derivative (which is also known as an ashless dispersant).

The present invention has been made on the above-mentioned inventors'new finding.

It has been further found out that the lubricating oil compositionscontaining the above-mentioned molybdenum complex of a basicnitrogen-containing compound but not containing the succinimide or itsderivative (and further not containing the succinate or its derivative)show improved resistance to oxidation at high temperatures but the oilcompositions show little reduction of the friction coefficient.

Accordingly, the present invention provides a lubricating oilcomposition for lubricating internal combustion engines which has asulfur content of 0.5 wt. % or less and which comprises a base oilhaving a lubricating viscosity and at least the following additivecomponents a) to e):

a) an ashless dispersant in an amount of 0.5-5.0 wt. % which comprisesan alkenyl- or alkyl-succinimide or a derivative thereof;

b) an ashless dispersant in an amount of 0.5-5.0 wt. % which comprisesan alkenyl- or alkyl-succinic ester or a derivative thereof;

c) a molybdenum complex of a basic nitrogen-containing compound in anamount of 50-1,200 wt.ppm in terms of a molybdenum content thereof;

d) an alkaline earth metal-containing detergent having a TBN in therange of 10 to 400 mg KOH/g in an amount of 0.05-1.0 wt. % in terms ofan alkaline earth metal content thereof; and

e) an anti-wear agent in an amount of 0.05-5.0 wt. %,

wherein the anti-wear agent is a sulfur-containing anti-wear agentselected from a sulfurized olefin, a polysulfide compound, a sulfurizedester, a sulfurized alcohol, a sulfurized amide, a sulfurized oil/fat,an ashless dithiocarbamate, a metal (other than molybdenum)dithiocarbamate, a mercapto thiadiazole, a mercapto benzothiadiazole,and a mercapto thiazoline, or a phosphorus-containing anti-wear agentselected from a phosphoric acid ester, a phosphorus acid ester, athiophosphoric acid ester, or amine salts thereof, zincdialkyldithiophosphate, zinc dialkyldithiophosphate and zincdialkylphosphate.

The present invention further provides a method for operating internalengines mounted to land-travelling vehicles using a fuel having a sulfurcontent of 0.001 wt. % or less under lubrication with theabove-mentioned lubricating oil composition according to the invention.

Preferred embodiments of the invention are stated below.

(1) The lubricating oil composition has the sulfur content in the rangeof 0.01 to 0.2 wt. %.

(2) The lubricating oil composition has the sulfur content in the rangeof 0.05 to 0.12 wt. %.

(3) The lubricating oil composition is a multi-grade engine oil furthercomprising a viscosity index improver, whereby being classified to 0W5,0W10, 0W15, 0W20, 0W30, 5W20, 5W30, 10W20, or 10W30.

(4) The lubricating oil composition contains the component a), namely,an alkenyl- or alkyl-succinimide or a derivative thereof, in an amountof 1.0-4.0 wt. %.

(5) The lubricating oil composition contains the component b), namely,an alkenyl- or alkyl-succinic ester or a derivative thereof, iscontained in an amount of 1.0-4.0 wt. %.

(6) The lubricating oil composition wherein a ratio of the component a)to the component b) is in the range of 1:4 to 4:1.

(7) The component c), namely, a molybdenum complex of a basicnitrogen-containing compound, contains a sulfur not larger than 1 wt. %.

(8) The component c), namely, the molybdenum complex of a basicnitrogen-containing compound, contains a sulfur in the range of 0.05 to0.5 wt. %.

(9) The component c) is a molybdenum complex of a succinimide.

(10) The component d) comprises an alkaline earth metal salicylate.

(11) The anti-wear agent is a zinc dialkyldithiophosphate.

(12) The lubricating oil composition further comprises an oxidationinhibitor.

(13) The lubricating oil composition is used for lubricating internalcombustion engines operated using a fuel oil having a sulfur content of0.001 wt. % or less.

The lubricating oil composition of the invention contains a molybdenumcomplex of a basic nitrogen-containing compound (serving as afriction-modifier) in combination of a mixture of known dispersants,that is, an alkenyl- or alkyl-succinimide or a derivative thereof and analkenyl- or alkyl-succinic acid ester (succinate) or a derivativethereof both in specific amounts. Therefore, the lubricating oilcomposition can be prepared to contain sulfur at a low level.Nevertheless, the lubricating oil composition of the invention shows aprominently good friction-reducing function.

DETAILED DESCRIPTION OF INVENTION

The base oil and additives employable for the preparation of thelubricating oil composition of the invention are described below in moredetail.

Base Oil

The base oil generally is a mineral oil or a synthetic oil showing akinematic viscosity of 2 to 50 mm²/s at 100° C. There are no specificlimitations on the natures and other properties of the mineral oil andsynthetic oil. However, the sulfur content of the base oil should be 0.1wt. % or less. The sulfur content preferably is 0.03 wt. % or less, andmore preferably is 0.005 wt. % or less.

The mineral oil preferably is an oil which is obtained by processing alubricating oil distillate of a mineral oil by solvent refining,hydrogenation, or their combination. Particularly preferred is a highlyhydrogenated refined oil (corresponding to a hydrocracked oil, typicallyhas a viscosity index of 100 to 150, an aromatic component content of 5wt. % or less, a nitrogen content of 50 wt.ppm or less, and a sulfurcontent of 50 wt. ppm or less. Particularly preferred is a highviscosity index base oil for instance having a viscosity index of140-160 which is obtained by hydroisomerization of slack wax or GTL wax(Gas-To-Liquid).

Examples of the synthetic oils (synthetic lubricating base oils) includepoly-α-olefin such as a polymerized compound of α-olefin having 3 to 12carbon atoms; a dialkyl ester of a di-basic acid such as sebacic acid,azelaic acid or adipic acid and an alcohol having 4 to 18 carbon atoms,typically dioctyl sebacate; a polyol ester which is an ester of1,1,1-trimethylolpropane or pentaerythritol and a mono-basic acid having3 to 18 carbon atoms; and alkylbenzene having an alkyl group of 9 to 40carbon atoms. The synthetic oil generally contains essentially nosulfur, shows good stability to oxidation and good heat resistance, andgives less residual carbon and soot when it is burned. Therefore, thesynthetic oil is preferably employed for the lubricating oil compositionof the invention. Particularly preferred is poly-α-olefin, from theviewpoint of the object of the invention.

Each of the mineral oil and synthetic oil can be employed singly. Ifdesired, however, two or more mineral oils can be employed incombination, and two or more synthetic oils can be employed incombination. The mineral oil and synthetic oil can be employed incombination at an optional ratio.

Ashless Dispersant

The lubricating oil composition of the invention comprises at least twodifferent ashless dispersants, that is, an alkenyl- or alkyl-succinimideor a derivative thereof (component a) and an alkenyl- or alkyl-succinicester or a derivative thereof (component b), in an amount of 0.5-5.0 wt.%, preferably 1.0-4.0 wt. %, for each (the amount is based on the totalamount of the lubricating oil composition). The component a) andcomponent b) are preferably contained in a weight ratio of 1:4 to 4:1,more preferably 1:2 to 2:1.

The component a), namely, the alkenyl- or alkyl-succinimide or aderivative thereof, may be a known alkenyl- or alkyl-succinimide or aderivative thereof. For example, the alkenyl- or alkyl-succinimidederived from a polyolefin or its derivative may be employed. Arepresentative succinimide can be prepared by the reaction betweensuccinic anhydride substituted with a high molecular weight alkenyl oralkyl and a polyalkylene polyamine containing average 4 to 10(preferably 5 to 7) nitrogen atoms in one molecule. The high molecularweight alkenyl or alkyl is preferably derived from a polybutene(particularly a high reactivity polybutene having a vinylidene terminal)having a number average molecular weight of about 900 to 3,000.

The process for obtaining the polybutenyl-succinic anhydride by thereaction of polybutene and maleic anhydride is generally performed bythe chlorination process using a chloride compound. The chlorinationprocess is advantageous in its reaction yield. However, the reactionproduct obtained by the chlorination process contains a large amount(for instance, approx. 2,000-3,000 wt.ppm) of chlorine. In contrast, ifthe thermal reaction process using no chloride compound is employed, thereaction product contains only an extremely small chlorine (forinstance, 0-30 ppm). Accordingly, it is preferred that the succinimidebe prepared from a polybutenyl succinic anhydride which is produced bythe thermal reaction, whereby the chlorine content of the resultingsuccinimide is in the range of 0 to 30 wt.ppm. The succinimide can bereacted with boric acid, alcohol, aldehyde, ketone, alkylphenol, cycliccarbonate, organic acid or the like, to give a modified succinimide.Particularly, a borated alkenyl(or alkyl)-succinimide which is obtainedby the reaction with boric acid or a boron compound is advantageous fromthe viewpoints of thermal and oxidation stability.

The component b), namely, the alkenyl- or alkyl-succinic ester or aderivative thereof, may be a known alkenyl- or alkyl-succinic ester or aderivative thereof. For example, the alkenyl- or alkyl-succinic esterderived from a polyolefin or its derivative may be employed. Arepresentative succinic ester can be prepared by the reaction betweensuccinic anhydride substituted with a high molecular weight alkenyl oralkyl and an alcohol containing average 1 to 6 hydroxyl groups in onemolecule. The high molecular weight alkenyl or alkyl is preferablyderived from a polybutene (particularly a high reactivity polybutenehaving a vinylidene terminal) having a number average molecular weightof about 900 to 3,000.

The lubricating oil composition of the invention may contain otherashless dispersants such as an alkenylbenzylamine ashless dispersant inaddition to the ashless dispersant a) (i.e., component a)) and ashlessdispersant b) (i.e., component b)).

Molybdenum Complex of Basic Nitrogen-Containing Compound

The lubricating oil composition of the invention further comprises amolybdenum complex of a basic nitrogen-containing compound in an amountof 50-1,200 wt.ppm in terms of a molybdenum content thereof. Thismolybdenum complex is already known as a multifunctional additive. Themolybdenum complex of a basic nitrogen-containing compound may contain asmall amount of sulfur.

The molybdenum complex of basic nitrogen-containing compound functionsmainly as an oxidation inhibitor, an anti-wear agent or a frictionmodifier in the lubricating oil composition.

A preferred molybdenum complex of a basic nitrogen-containing compoundis an oxymolybdenum complex produced by reaction of an acidic molybdenumcompound and a basic nitrogen-containing compound. Examples of theacidic molybdenum compounds include molybdic acid, ammonium molybdate,and alkali metal salts of molybdate. Examples of the basicnitrogen-containing compounds include succinimide, carboxylic amide,hydrocarbylamine and a Mannich basic compound. Examples of themolybdenum complex of basic nitrogen-containing compounds include amolybdenum complex of succinimide and a molybdenum complex of asecondary aliphatic amine.

The molybdenum complex of a basic nitrogen-containing compound can beemployed after reaction with a small amount of sulfur or asulfur-containing compound. Specifically, a reaction product obtained bysulfurization of a molybdenum complex of succinimide at lowtemperatures. The reaction product preferably contains sulfur in arelatively small amount such as 10 wt. % or less. The sulfoxymolybdenumcomplex can be prepared in the manner described in Examples C to H ofU.S. Pat. No. 6,562,765 B.

The lubricating oil composition of the invention can further containother molybdenum-containing compounds such as sulfoxymolybdenumdithiocarbamate, sulfoxymolybdenum dithiophosphate, and oxymolybdenummonoglyceride.

Alkaline Earth Metal-Containing Detergent

The lubricating oil composition of the invention further comprises analkaline earth metal-containing detergent having a TBN in the range of10 to 400 mg KOH/g in an amount of 0.05-1.0 wt. % in terms of analkaline earth metal content thereof. The known alkaline earthmetal-containing detergent having a TBN in the range of 10 to 400 mgKOH/g is employable. Preferred is a calcium alkylsalicylate detergent.The calcium alkylsalicylate detergent preferably contains an organicacid salt in an amount of 0.2-7 wt. α %, more preferably 0.5-5 wt. %,most preferably 1.0-3 wt. %. The calcium alkylsalicylate detergentpreferably comprises an unsulfurized calcium alkylsalicylate detergenthaving an alkyl group containing 14-18 carbon atoms. The unsulfurizedcalcium alkylsalicylate detergent having an alkyl group containing 14-18carbon atoms can be employed in combination with an unsulfurized calciumalkyl-salicylate detergent having an alkyl group containing 20-28 carbonatoms. In the specification, the unsulfurized calcium alkylsalicylatedetergent having an alkyl group containing 14-18 carbon atoms means anunsulfurized calcium alkylsalicylate detergent having an alkyl group inwhich 90 mole % or more of the alkyl groups contain 14-18 carbon atoms.The unsulfurized calcium alkylsalicylate detergent having an alkyl groupcontaining 20-28 carbon atoms has the same meaning, that is, theunsulfurized calcium alkylsalicylate detergent having an alkyl groupcontaining 20-28 carbon atoms means an unsulfurized calciumalkylsalicylate detergent having an alkyl group in which 90 mole % ormore of the alkyl groups contain 20-28 carbon atoms.

The unsulfurized calcium alkylsalicylate preferably is a calcium salt ofan alkylsalicylic acid which can be prepared from an alkylphenol (whichis produced by the reaction of α-olefin having the desired number ofcarbon atoms and phenol) by the Kolbe-Schmidt reaction. Generally, thecalcium salt of an alkylsalicylic acid can be overbased by the use oflime and carbon dioxide, to give an overbased calcium salicylate.

The calcium salt of an alkylsalicylate can be prepared by the steps ofneutralizing phenol to give its calcium salt and subsequent carbonation.

The calcium alkylsalicylate detergent is preferably incorporated intothe lubricating oil composition under such conditions that the amount ofthe organic acid salt (contained in the calcium alkylsalicylatedetergent) is in the range of 0.2 to 7 wt. %, preferably 0.5 to 5 wt. %,more preferably 1.0 to 3 wt. %, based on the total amount of thelubricating oil composition. In more detail, the calcium alkylsalicylatedetergent is an oily dispersion which comprises an organic acid metalsalt (generally called a soap component) and basic inorganic saltmicro-particles (e.g., calcium carbonate particles) aggregated aroundthe organic acid metal salt in an oily medium. Generally, the dispersioncontains the oily medium in an amount of 30 to 50 wt. %. Even if theamount of the calcium alkylsalicylate detergent is reduced, the hightemperature detergency (i.e., power for keeping the inside space of anengine operated at high temperatures) of the lubricating oil compositiondoes not substantially lower provided that the amount of the organicacid metal salt is kept at or above a certain level.

The alkaline earth metal-containing detergent can be an alkaline earthmetal salt of an organic acid having a carbon-nitrogen bonding or aphenol derivative. This detergent can have a high TBN regardless of asulfated ash content, if it is treated with an amine compound. The aminecompound brings a base number originating from the basic nitrogen atoms.For example, a metal salt of an aminocarboxylic acid can be employed.Particularly preferred is an unsulfurized alkylphenate (i.e., alkalineearth metal salt) having a Mannich base structure. This compound can beobtained by Mannich reaction using an alkylphenol, formaldehyde, and anamine or an amine compound to give an aminomethylated phenol, andsubsequent neutralization of the reaction product by a base such ascalcium hydroxide.

Alternatively, the alkaline earth metal-containing detergent can be analkaline earth metal salt of a sulfonic acid (i.e., sulfonate) obtainedfrom a petroleum sulfonic acid, an alkylbenzenesulfonic acid, or analkyltoluenesulfonic acid.

Alternatively, the alkaline earth metal-containing detergent can be asulfurized phenate, namely, an alkaline earth metal salt of a sulfurizedalkyl phenol

Anti-Wear Agent

The lubricating oil composition of the invention further comprises ananti-wear agent (component e)) in an amount of 0.05-5.0 wt. %. There areno specific limitation on the anti-wear agent, and the knownsulfur-containing anti-wear agent as well as the knownphosphorus-containing anti-wear agent can be employed. Examples of thesulfur-containing anti-wear agents include sulfurized olefin,polysulfide compound, sulfurized ester, sulfurized alcohol, sulfurizedamide, sulfurized oil/fat, ashless dithiocarbamate, metal (other thanmolybdenum) dithiocarbamate, mercapto thiadiazole, mercaptobenzothiadiazole, and mercapto thiazoline. Examples of thephosphorus-containing anti-wear agents include phosphoric acid ester,phosphorus acid ester, thiophosphoric acid ester, their amine salts, andtheir metal salts such as zinc dialkyldithiophosphate, zincdialkyldithiophosphate, and zinc dialkylphosphate.

The anti-wear agent, i.e., component e), preferably is zincdialkyldithiophosphate or zinc dihydrocarbylphosphate. These zincphosphate anti-wear agents are well known and easily produced by theknown procedures. The zinc phosphate anti-wear agent can be employed inan amount of 0.01-0.12 wt. %, but preferably in an amount of 0.01-0.06wt. %, in consideration of the desired low phosphorus content and sulfurcontent.

The zinc dialkyldithiophosphate preferably contains an alkyl grouphaving 3-18 carbon atoms or an alkylaryl group having C₃₋₁₈ alkyl group.Most preferred is a zinc dialkyldithiophosphate containing an alkylgroup derived from a secondary alcohol having 3-18 carbon atoms or azinc dialkyldithiophosphate containing a mixture of alkyl groups derivedfrom a mixture of a primary alcohol having 3-18 carbon atoms and asecondary alcohol having 3-18 carbon atoms. Both are particularlyeffective for reducing wear. A zinc dialkyldithiophosphate derived froma primary alcohol shows high thermal resistance. These zincdialkyldithiophosphates can be employed alone or in combination in theform of a mixture mainly comprising one derived from the secondaryalcohol and/or one derived from the primary alcohol.

Oxidation Inhibitor

The lubricating oil composition of the invention preferably contains anoxidation inhibitor in an amount of 0.01-5 wt. %, preferably 0.1-3 wt.%. The oxidation inhibitor can be the phenolic oxidation inhibitor andthe amine oxidation inhibitor. A lubricating oil composition having alow sulfated ash content, a low phosphorus content and a low sulfurcontent is apt to show a relatively low high-temperature detergency,oxidation-inhibiting performance and anti-wear performance due todecrease of the amounts of the metal-containing detergent and zincdithiophosphate. Therefore, the lubricating oil composition of theinvention preferably contains an oxidation inhibitor so as to keep theseperformances high.

Preferred oxidation inhibitors are a diarylamine oxidation inhibitor anda hindered phenol oxidation inhibitor. These oxidation inhibitors alsofunction for improving high-temperature detergency. The diarylamineoxidation inhibitor is advantageous in giving a base number originatingfrom the nitrogen atoms. The hindered phenol oxidation inhibitor isadvantageous in producing no NO_(x) gas.

Examples of the hindered phenol oxidation inhibitors include2,6-di-t-butyl-p-cresol, 4,4′-methylenebis(2,6-di-t-butylphenol),4,4′-methylenebis(6-t-butyl-o-cresol),4,4′-isopropylidenebis(2,6-di-t-butylphenol),4,4′-bis(2,6-di-t-butylphenol),2,2′-methylenebis(4-methyl-6-t-butylphenol),4,4′-thiobis(2-methyl-6-t-butylphenol),2,2-thio-diethylenebis[3-(3,5-di-t-butyl-4-hydoxyphenyl)propionate],octyl 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, octadecyl3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, and octyl3-(3,54-butyl-4-hydroxy-3-methylphenyl)propionate.

Examples of the diarylamine oxidation inhibitors includealkyldiphenylamine having a mixture of alkyl groups of 4 to 9 carbonatoms, p,p′-dioctyldiphenylamine, phenyl-α-naphthylamine,phenyl-α-naphthylamine, alkylated β-naphthylamine, and alkylatedphenyl-α-naphthylamine.

Each of the hindered phenol oxidation inhibitor and diarylamineoxidation inhibitor can be employed alone or in combination. If desired,other oil soluble oxidation inhibitors can be employed in combinationwith the above-mentioned oxidation inhibitor(s).

Other Additives

The lubricating oil composition of the invention may further contain analkali metal borate hydrate for increasing stability at hightemperatures and a base number. A representative alkali metal boratehydrate can be prepared in the manner described in U.S. Pat. Nos.3,929,650 and 4,089,790. For example, the alkali metal borate hydratecan be in the form of a dispersion containing micro-particles of analkali metal borate hydrate which is produced by carbonizing a neutralalkali metal or alkaline earth metal sulfonate in the presence of analkali metal hydroxide to yield an overbased sulfonate and reacting theoverbased sulfonate with boric acid. In the carbonizing procedure, it isdesired to incorporate an ashless dispersant such as succinimide intothe reaction mixture. The alkali metal preferably is potassium orsodium. The alkali metal borate hydrate can be a dispersion of KB₃O₅H₂Oparticles having a particle size of about 0.3 m or less which isdispersed in the presence of a neutral calcium sulfonate andsuccinimide. From the viewpoint of resistance to hydrolysis, potassiumis preferably replaced with sodium.

The lubricating oil composition of the invention preferably contains aviscosity index improver in an amount of 20 wt. % or less, preferably 1to 20 wt. %. Examples of the viscosity index improvers are polymers suchas polyalkyl methacrylate, ethylene-propylene copolymer,styrene-butadiene copolymer, and polyisoprene. A dispersant viscosityindex improver and a multi-functional viscosity index improver which areproduced by providing dispersing properties to the above-mentionedpolymer are preferably employed. The viscosity index improvers can beused alone or in combination.

The lubricating oil composition of the invention may further contain asmall amount of various auxiliary additives. Examples of the auxiliaryadditives are described below.

benzotriazol compounds and thiadiazol compounds functioning as metaldeactivating agent; nonionic polyoxyalkylene surface active agents suchas polyoxyethylenealkylphenyl ether and copolymers of ethylene oxide andpropylene oxide functioning as an anti-rust agent and ananti-emulsifying agent. In addition, various compounds functioning as ananti-foaming agent and a pour point depressant can be incorporated.

The above-mentioned auxiliary additives may be incorporated into thelubricating oil composition in an amount of 3 wt. % or less,particularly in an amount of 0.001-3 wt. %.

EXAMPLES (1) Preparation of Lubricating Oil Composition (Test Oil)

Lubricating oil compositions having a low sulfur content (0.5 wt. % orless) for evaluation on the oil performance were prepared in thebelow-described manner using the below-mentioned base oil and additives.The lubricating oil compositions were formulated to show a viscositygrade (SAE viscosity grade) of 5W30 by addition of a viscosity indeximprover.

(2) Base Oil and Additives 1) Base Oil (Mixture of the Below-MentionedBase Stock A and Base Stock B in the Weight Ratio of 46:54)

Base stock A: Hydrocracked base stock showing a kinematic viscosity of6.4 mm²/s (at 100° C.), a viscosity index of 132, a saturated componentcontent of 92 wt. % and a sulfur content of less than 0.001 wt. %.

Base stock B: Hydrocracked base stock showing a kinematic viscosity of4.1 mm²/s (at 100° C.), a viscosity index of 127, a saturated componentcontent of 92 wt. % and a sulfur content of less than 0.001 wt. %.

2) Additives

Ashless dispersant A: Ethylene carbonate post-treated succinimidedispersant (nitrogen content: 0.85 wt. %) prepared by the steps ofthermally reacting polybutene (number average molecular weight: about2,200) and maleic anhydride to give polyisobutenylsuccinic anhydride,reacting about 2 moles of the polyisobutenyl-succinic anhydride andpolyalkylene polyamine containing 6.5 nitrogen atoms (average) in onemolecule to give a bis-type succinimide, and finally treating thebis-type succinimide with ethylene carbonate under reaction.

Ashless dispersant B: Alkenylsuccinic ester (TAN: 5 mg KOH/g) preparedby the steps of thermally reacting polybutene (number average molecularweight: about 1,000) and maleic anhydride to give polyisobutenylsuccinicanhydride, reacting about 1.14 moles of the polyisobutenylsuccinicanhydride and one mole of pentaerythritol.

Mo complex of succinimide: A product (Mo content: 4.5 wt. %, N content:2.1 wt. %, S content: 0.16 wt. %) obtained by the steps of reacting amono-type alkenyl-succinimide derived from polyisobutene having a numberaverage molecular weight of about 1,000 and molybdic acid to give amolybdenum complex and reacting the molybdenum complex with a smallamount of sulfur.

MoDTC: Sulfoxymolybdenum dialkyldithiocarbamate (Mo content: 10 wt. %, Scontent: 11 wt. %)

Ca-containing detergent A: Calcium monoalkylsalicylate (alkyl containingabout 14-18 carbon atoms, overbase degree: 2.0, Ca content: 6.2 wt. %, Scontent: 0.12 wt. %)

Ca-containing detergent B: Calcium monoalkylsalicylate (alkyl containingabout 20-28 carbon atoms, overbase degree: 8.2, Ca content: 11.4 wt. %,S content: 0.17 wt. %)

Ca-containing detergent C: Calcium sulfonate (neutral type, Ca content:2.4 wt. %, sulfur content: 2.7 wt. %)

ZnDTP: zinc dialkyldithiophosphate (P content: 7.2 wt. %, S content:14.4 wt. %, derived from a mixture of a secondary alcohol having 4carbon atoms and a secondary alcohol having 6 carbon atoms)

Oxidation inhibitor: Mixture of dialkyldiphenylamine oxidation inhibitorand hindered phenol propionate oxidation inhibitor.

Viscosity index improver (VII): Non-dispersant type ethylene-propylenecopolymer (Paratone 8057)

(3) Determination of Friction Coefficient

The friction coefficient was determined by means of High FrequencyReciprocating Rig (HFRR) under the below-described conditions:

Oil temperature: 105° C., Load: 400 g, Friction length: 1,000 m,Reciprocating frequency: 20 Hz, Test Period: one hour.

(4) Evaluation of Inhibition of Production of High-Temperature Deposit

The power inhibiting production of a high-temperature deposit (Deposit)was evaluated by means a hot tube tester (HTT) under the below describedconditions:

In a heater block was placed vertically a glass tube (inner diameter: 2mm). The test oil and air were supplied into the tube from its bottom atsupply rates of 0.31 cc/h and 10 cc/min., respectively. These procedureswere performed under keeping the temperature of the heater element at280° C. for 16 hours. Thereafter, the glass tube was taken out from theheater block, washed with petroleum ether, and dried. The depositattached to the inner surface of the glass tube was observed and markedaccording to the merit rating.

The merit rating was done in the known manner on the basis of 10 points.The higher point means that the power of inhibiting production ofdeposit is high.

(5) Compositions of the Test Oils and Results of Determination andEvaluation

The test results of the test oils are set forth in Table 1 below.

TABLE 1 Base oil/ Additives Example 1 Com. Ex. 1 Com. Ex. 2 Ashlessdisper- 3.0 5.5 3.0 sant A (wt. %) Ashless disper- 2.5 — 2.5 sant B (wt.%) Succinimide Mo- 220 220 — complex (Mo wt. ppm) MoDTC — — 220 (Mo wt.ppm) Ca detergent A 0.19 0.19 0.19 (Ca wt. %) Ca detergent B 0.06 0.060.06 (Ca wt. %) Ca detergent C 0.01 0.01 0.01 (Ca wt. %) ZnDTP (P wt. %)0.04 0.04 0.04 Oxidation in- 1.0 1.0 1.0 hibitor (wt. %) VII (wt. %) 5.05.0 5.0 Base oil remainder remainder remainder S content (wt. %) 0.110.11 0.13 Friction 0.05 0.12 0.05 coefficient HTT merit point 7.5 7.55.5

Remarks:

The test results set forth in Table 1 indicate the following:

The lubricating oil composition of the invention comprisingsuccinimide-Mo complex, succinimide dispersant (Ashless dispersant A)and succinic ester dispersant (Ashless dispersant B) in combination(Test oil of Example 1) shows the same high-temperature depositproduction-inhibiting power but markedly low friction coefficient ascompared with the lubricating oil composition (Test oil of ComparisonExample 1) comprising the succinimide-Mo complex and succinimidedispersant (Ashless dispersant A) in combination but containing nosuccinic ester dispersant (Ashless dispersant B). Test oil of ComparisonExample 2 prepared by replacing the succinimide-Mo complex with the sameamount of MoDTC (sulfoxy molybdenum dialkyldithiocarbamate) shows thesame friction coefficient but poor power of inhibiting production ofhigh-temperature deposit.

What is claimed is:
 1. A lubricating oil composition for lubricatinginternal combustion engines which has a sulfur content of 0.5 wt. % orless and which comprises a base oil having a lubricating viscosity andat least the following additive components a) to e): a) an ashlessdispersant in an amount of 0.5-5.0 wt. % which comprises an alkenyl- oralkyl-succinimide or a derivative thereof; b) an ashless dispersant inan amount of 0.5-5.0 wt. % which comprises an alkenyl- or alkyl-succinicester or a derivative thereof; c) a molybdenum complex of a basicnitrogen-containing compound in an amount of 50-1,200 wt.ppm in terms ofa molybdenum content thereof; d) an alkaline earth metal-containingdetergent having a TBN in the range of 10 to 400 mg KOH/g in an amountof 0.05-1.0 wt. % in terms of an alkaline earth metal content thereof;and e) an anti-wear agent in an amount of 0.05-5.0 wt. %, wherein theanti-wear agent is a sulfur-containing anti-wear agent selected from asulfurized olefin, a polysulfide compound, a sulfurized ester, asulfurized alcohol, a sulfurized amide, a sulfurized oil/fat, an ashlessdithiocarbamate, a metal (other than molybdenum) dithiocarbamate, amercapto thiadiazole, a mercapto benzothiadiazole, and a mercaptothiazoline, or a phosphorus-containing anti-wear agent selected from aphosphoric acid ester, a phosphorus acid ester, a thiophosphoric acidester, or amine salts thereof, zinc dialkyldithiophosphate, zincdialkyldithiophosphate and zinc dialkylphosphate.
 2. The lubricating oilcomposition of claim 1, wherein the sulfur content is in the range of0.01 to 0.2 wt. %.
 3. The lubricating oil composition of claim 1,wherein the sulfur content is in the range of 0.05 to 0.12 wt. %.
 4. Thelubricating oil composition of claim 1, which is a multi-grade engineoil further comprising a viscosity index improver, whereby beingclassified to 0W5, 0W10, 0W15, 0W20, 0W30, 5W20, 5W30, 10W20, or 10W30.5. The lubricating oil composition of claim 1, wherein the component a)is contained in an amount of 1.0-4.0 wt. %.
 6. The lubricating oilcomposition of claim 1, wherein the component b) is contained in anamount of 1.0-4.0 wt. %.
 7. The lubricating oil composition of claim 1,wherein the ratio of the component a) to the component b) is in therange of 1:4 to 4:1.
 8. The lubricating oil composition of claim 1,wherein the component c) contains sulfur not larger than 1 wt. %.
 9. Thelubricating oil composition of claim 1, wherein the component c)contains sulfur in the range of 0.05 to 0.5 wt. %.
 10. The lubricatingoil composition of claim 1, wherein the component c) is a molybdenumcomplex of a succinimide.
 11. The lubricating oil composition of claim1, wherein the component d) comprises an alkaline earth metalsalicylate.
 12. The lubricating oil composition of claim 1, wherein theanti-wear agent is a zinc dialkyldithiophosphate.
 13. The lubricatingoil composition of claim 1, which further comprises an oxidationinhibitor.
 14. The lubricating oil composition of claim 1, which isemployed for lubricating internal combustion engines operated using afuel oil having a sulfur content of 0.001 wt. % or less.
 15. A methodfor operating internal engines mounted to land-travelling vehicles usinga fuel having a sulfur content of 0.001 wt. % or less under lubricationwith the lubricating oil composition of claim 1.